Fix installation of the systemd unit

The path provided to ECM installed to $PREFIX/lib/systemd, which is
incorrect both for system units (should be $PREFIX/lib/systemd/system)
and user units (should be $PREFIX/lib/systemd/user).

As kactivitymanagerd is handled per-user, install it in the systemd
user unit directory.

CCMAIL: henri.chain@enioka.com